The present invention relates to the field of optical communications systems and particularly to a method for reduction or elimination of timing jitter and amplitude jitter occurring in transmission of Return-to-Zero (RZ) modulated pulses by use of optimum amount of pre-chirp.
Transmission of optical pulses based on RZ modulation is emerging as the best choice in high bit rate and/or long distance systems. However, the pulses suffer from nonlinear intra-channel effects, which lead to timing jitter and amplitude jitter. Timimg jitter and amplitude jitter weaken the performance and limit the maximum capacity of each channel.
Dispersion describes how a signal is distorted due to the various frequency components of the signal having different propagation characteristics. Specifically, dispersion is the degree of scattering in the light beam as it travels along a fiber span. Dispersion can also be caused by the frequency dependence of the group velocity of a light signal propagating inside a fiber.
The intricate interplay of nonlinearity and dispersion acting on pulses in optical fibers continues to challenge the conventional wisdom and established intuition. One example is the idea that short duty-cycle RZ transmission in dispersive fibers is able to combat the detrimental effects of fiber nonlinearity. Due to their short width, the pulses disperse rapidly, spreading in time over hundreds or thousands of bits. Theory, simulations and experiments in the prior art uniformly show that with shorter pulses the nonlinear impairments are reduced. This may seem somewhat counter-intuitive since the reduction of the pulse width is inevitably accompanied by an increase in the pulse peak power and an increase in the impact of self-phase modulation (SPM) may be expected. SPM causes compression in the pulse. The reason for the reduction is not merely that the individual peak power is reduced by dispersion. In a random bit sequence the intensity pattern of the interfering pulses contains spikes that are of the same order of magnitude as the input peak power. The reduced peak power is, therefore, not a viable explanation. Rather, the mechanism for the tolerance towards nonlinear impairments relies on the fact that the intensity pattern changes very rapidly. Thus, the accumulated effect of the instantaneous nonlinearity tends to get averaged out and SPM and nonlinear pulse interaction is reduced even though the pulses spread over hundreads of neighboring time slots. The concept of spreading the pulses as far as possible and as quickly as possible in the time domain, creating a rapidly varying intensity pattern, in order to combat the impact of nonlinearity, represent such a big shift from standard dispersion managed approaches that a specific term xe2x80x9ctedon-transmissionxe2x80x9d has been coined to represent this scheme.
System penalties are generated in the form of timing and amplitude jitter, which limit the performance of such systems. It may be useful to note that the scheme presented herein is fundamentally different from schemes, which rely on soliton transmission where the pulses usually do not spread over more than tens of bits.
Analysis of the nonlinear pulse interaction in systems based on highly dispersed optical pulses provides estimates of timing and amplitude jitter. The pulse streams are both coherent and non-coherent. Analysis of the nonlinear intra-channel effects indicate that the non-linear effects possess a symmetry when pre-chirped pulses are launched. System penalties reduce montonically with decreasing pulse width and with increasing fiber dispersion. Proper dispersion pre-compensation can result in a significant reduction of the nonlinear impairments. Optimal pre-compensation can be determined analytically.
It is, therefore, an object of the present invention to minimize timing jitter by injecting the proper amount of pre-chirp into the communications link.
A further object of the present invention is to minimize amplitude jitter by injecting the proper amount of pre-chirp into the communications link.